Notes from the Alliance’s 2018 Phosphorus Modeling Workshop

The Phosphorus Field-to-Watershed (P-F2W) Modeling Workshop brought 16 researchers and policy experts to Columbus, Ohio, on August 23rd and 24th. The group discussed soil test phosphorus measurements and their relationship to fertilizer application recommendations, status and trends of edge-of-field and in-stream water quality observations for the Maumee River basin (a tributary of Lake Erie and a major contributor of P loadings to the lake), and existing models to simulate soil phosphorus and their application in the Maumee basin.

Chad Penn (USDA-ARS) discussed how soil test phosphorus measurements are used to guide fertilizer application rates. Dr. Penn suggested that these recommendations provide good “ball-park” application recommendations but that they may be better tailored to the soil texture, the crop being grown, and other site conditions. He suggested that these improvements should improve agricultural efficiency while decreasing P loss from fields.

Laura Johnson (Heidelberg University) and Kevin King (USDA-ARS) presented their efforts to assess phosphorus loadings to Lake Erie using in-stream and edge-of-field monitoring networks, respectively. In summarizing long-term, in-stream observational data, Dr. Johnson showed that current, flow-weighted, mean dissolved P concentrations are consistent with historical trends – implying that recent efforts to decrease loadings to the Lake have not been successful, at least to this point in time. Dr. Johnson also suggested that the continuing problem may be partly attributed to increased soil P stratification (i.e. higher P concentrations at the top with lower concentration at greater depths) as a consequence of reduced tillage. Dr. King then described his network of edge-of-field sites that measure P loss from fields in surface and tile flow and noted that most of the monitored fields do not show excessive quantities of P export. Dr. King additionally discussed the impact of various best management practices as applied to these sites, reviewing the relative success of each.

Peter Vadas (USDA-ARS) and Margaret Kalcic (Ohio State University) discussed their work to simulate soil phosphorus dynamics using empirical and process-based models. Dr. Vadas reviewed the evolution of models that simulate soil phosphorus dynamics in agricultural systems – beginning with the EPIC model, then moving to the APEX, SWAT and APLE models. He described the individual “pools” of phosphorus (i.e., labile, active and stabile mineral phosphorus, and organic phosphorus) and emphasized that the labile mineral pool can be initialized using common soil test phosphorus measurements. He then outlined previous work to validate SWAT’s phosphorus routines and introduced his continuing work with the APLE model. Dr. Kalcic provided an overview of the SWAT model and recent applications of that model in the Maumee watershed. She then introduced a new iteration of the model – wherein the most basic spatial unit of simulation approximates the size of agricultural fields – and her ongoing efforts to validate SWAT’s simulation of soil phosphorus dynamics, including phosphorus export through surface and tile flow. She also discussed several challenges in simulating phosphorus in the Maumee – namely problems with calibration when initializing labile P at higher values and with the insensitivity of modeled crop growth to soil phosphorus levels.

Finally, Tom Zimnicki provided a summary of a workshop hosted by the Cooperative Institute for Great Lakes Research at the University of Michigan, Ann Arbor, in July 2018. The workshop hosted a number of researchers and sought to improve hydrologic model representation of soil health as impacted by agricultural best management practices.

Following presentations, the group plotted a path forward. Three specific areas of investigation were identified for additional discussion: identifying whether legacy or incidental losses were the primary sources of P loadings to Lake Erie; coupling fertilizer production, particularly from recycled organics, and watershed hydrologic models; and identifying “hotspots” (i.e. locations of elevated P export) using hydrologic models. The meeting was concluded by discussing plans to collaborate in investigation of these specific topics.

The workshop was organised by the Sustainable Phosphorus Alliance. The workshop agenda and presentation slidedecks are available for download in PDF format in the sidebar to the event listing on this page. In addition, you can view video of these talks on our YouTube channel.

Grey Evenson is a postdoctoral fellow in the Department of Food, Agricultural and Biological Engineering at Ohio State University. Grey is a watershed scientist with an interest in applying models to improve environmental outcomes. Opinions expressed here do not necessarily reflect those of the Sustainable Phosphorus Alliance.